Method of improving the flow conditions in an axial-flow compressor, and axial-flow compressor for carrying out the method

ABSTRACT

In a method of improving the flow conditions on the suction side ( 21 ) of the blade ( 12 ) of an axial-flow compressor ( 10 ) provided for compressing air, in which method vortices running in the direction of flow are generated on the suction side ( 21 ), a marked improvement is achieved in a simple and reliable manner by the vortices being generated by local air jets ( 18 ) formed on the suction side ( 21 ).

TECHNICAL FIELD

The present invention relates to the field of compressor technology. It relates to a method of improving the flow conditions on the suction side of the blade of an axial-flow compressor provided for compressing air according to the preamble of claim 1. It also relates to an axial-flow compressor for carrying out the method.

PRIOR ART

The first stage of the compressor of a gas turbine normally works within the supersonic range. In order to further increase the pressure rise per stage, it is desirable to increase the pre-shock Mach number for a higher pressure ratio. The maximum pre-shock Mach number may increase up to about 1.5. The main problems associated with such high Mach numbers are a shock-induced separation of boundary layers and a very narrowly restricted working range of the compressor (“unique incidence condition”). Ways of improving the flow behavior for such high Mach numbers are therefore constantly looked for. The limiting pre-shock Mach number for operation without boundary layer separation is currently considered to be around 1.3.

In order to increase the resistance of the boundary layers against higher shock Mach numbers, the contour of the blades may be changed on the one hand (see, for example, U.S. Pat. No. 5,554,000 or U.S. Pat. No. 6,017,186), or control mechanisms for the boundary layers may be used on the other hand (see, for example, U.S. Pat. No. 5,598,990). The potential for adaptations of the blade contour appears to be largely exhausted nowadays. Therefore the control of the boundary layers is the most suitable way of increasing the working range and the pressure rise for a supersonic compressor. However, such control mechanisms for boundary layers must be of simple construction, so that they can be fitted into the rotating machine without any problems. In addition, the risk of parts of the mechanism being able to fall off and damage the following stages should be minimal. Finally, such systems should be easy to control, so that faults in the control do not lead to serious machine damage.

Especially in the field of external aerodynamics, e.g. in the case of wings, considerable research has been carried out with regard to methods of controlling shock or boundary layers. In this case, the research has concentrated initially on wing-shaped vortex generators on aircraft airfoils. It has led to such wing-shaped vortex generators being used as standard items in commercial aircraft (e.g. the Boeing 727) in order to improve the stability of the boundary layers in the zone of interaction with the shock. Research has shown (Pearcey et al., Inclined Air-Jets Used as Vortex Generators to Suppress Shock-induced Separation, AGARD Meeting, April 1993) how effectively the flow in the zone of interaction between shock and boundary layer can be stabilized using air-jet vortex generators (also see US-B1-6,302,360).

In the field of turbomachines, research with regard to air-jet vortex generators is much more limited. The use of air-jet vortex generators is mainly investigated with respect to the aerodynamics of turbines, since the blades are cooled there and thus the requisite air for the vortex generators is available anyway (J. P. Bons et al., The Fluid Mechanics of LPT Blade Separation Control Using Pulsed Jets, ASME 2001-GT-0190).

The possible use of vortex generators on the rotor of a supersonic compressor was not even mentioned in an overview of flow control from the year 2000 (W. K. Lord et al., Flow Control Opportunities in Gas Turbine Engines, AIAA June 2000 in Denver, AIAA 2000-2234). The fact that boundary-layer control by vortex generators in compressors was not mentioned here is certainly connected with the very small blade thickness, which does not allow larger flow mechanisms to be accommodated in the interior of the blade. In particular, this also means that the space for a plenum from which air would be provided for air-jet vortex generators is very restricted. Although wing-shaped vortex generators do not absolutely require control mechanisms, they have the great disadvantage that there is always the risk of separation from the surface with said control mechanisms, and this separation would then lead to considerable damage to the following stages.

DESCRIPTION OF THE INVENTION

The object of the invention is to specify a method and an axial-flow compressor with which the flow conditions on the suction side of the blades of the compressor can be improved with regard to the shock stabilization in a reliable and simple manner, in particular without the need for additional built-in components.

The object is achieved by all the features of claims 1 and 7 in their entirety. The essence of the invention consists in the fact that vortices running on the suction side in the direction of flow are generated by local air jets formed on the suction side. Due to the local air jets, the use of additional elements arranged on the blade surfaces is avoided and the risk of such elements falling off, with the resulting damage to following stages, is removed.

According to a preferred configuration of the method according to the invention, to form the local air jets, air having a higher pressure relative to the environment is discharged from first openings in the suction-side surface of the blade. It is thereby possible to avoid parts which project from the surface of the blade and adversely affect the flow.

The method according to the invention becomes especially simple if, according to a preferred development, the air having the higher pressure on the pressure side of the blade is extracted from the flow and is directed through the blade to the first openings in the suction-side surface of the blade. In this way, greatly simplified, automatic generation of air jets is realized, which is distinguished by a minimum space requirement and renders control means arranged outside the compressor unnecessary.

The method can be carried out especially effectively and with minimum cost if a second opening on the pressure side of the blade is in this case assigned to each first opening on the suction side of the blade, and if the air having the higher pressure is in each case extracted at the second opening and directed via passages in the blade to the associated first opening.

The generation of air jets can be further improved if, according to another configuration of the invention, to increase the pressure of the air extracted at the pressure side, the dynamic pressure associated with the flow is also utilized in addition to the static pressure prevailing on the pressure side, in which case a first passage in the blade is used in particular in order to utilize the dynamic pressure during the extraction of the air, this first passage being inclined in the direction of flow.

A preferred configuration of the axial-flow compressor according to the invention is characterized in that the first openings are each arranged in the vicinity of the leading edge of the blade, in that, as a source for air having a higher pressure, second openings on the pressure side of the blade are assigned to the first openings on the suction side of the blade, a second opening being assigned in particular to each first opening and being connected to the first opening via passages running transversely through the blade, and in that in each case a first passage is arranged in the interior of the blade behind the second opening, this first passage running in the direction of flow and enclosing an acute angle, in particular of about 20°, with the pressure-side surface of the blade.

In each case a second passage is preferably arranged in the interior of the blade behind the first opening, this second passage running in the direction of flow and enclosing an acute angle, in particular of about 60°, with the pressure-side surface of the blade, the second passage being tilted by an angle, preferably of about 45°, from the plane spanned by the first passage and the direction of flow. The first and second passages open into one another in the interior of the blade.

Further embodiments follow from the dependent claims.

BRIEF EXPLANATION OF THE FIGURES

The invention is to be explained in more detail below with reference to exemplary embodiments in connection with the drawing, in which:

FIG. 1 shows, in a plan view in the radial direction, a plurality of blades of a blade ring of an axial-flow compressor with air jets as vortex generators according to a preferred exemplary embodiment of the invention;

FIG. 2 shows, in a cross section, a preferred configuration of the vortex generators according to the invention with differently inclined connecting passages between pressure side and suction side of the blade;

FIG. 3 shows the course of the connecting passages from FIG. 2 as viewed in the direction of flow; and

FIG. 4 shows, in a side view, the suction side of a blade from FIG. 1 with the openings of the vortex generators, these openings being arranged in a radial row.

WAYS OF IMPLEMENTING THE INVENTION

The present invention comprises a method and a device for generating vortices, running in the direction of flow, by means of air jets on the blades of a supersonic compressor, as a result of which the flow conditions on the suction side of the blades are improved. The shock front can be stabilized as a result and the working range and the pressure ratio of the compressor can thus be increased. In addition, the method applied results in a suction effect on the pressure side of the blades, which improves the state of the boundary layer after the interaction with the shock wave on the pressure side.

A preferred exemplary embodiment of the invention is reproduced in FIGS. 1 to 4. In the radial plan view, FIG. 1 shows a plurality of blades 12 of a blade ring 11 of an axial-flow compressor 10. Provided the blades 12 are moving blades arranged on the rotor, these blades move in a direction of rotation 16. The blades 12 each have a leading edge 13 and a trailing edge 14 and also a pressure side 20 and a suction side 21. The axial direction of the axial-flow compressor 10 is designated by 15 in FIG. 1, and the flow which passes through between the blades 12 and can form a shock wave front 19 is designated by 17. To generate vortices, air jets 18 are now generated through the blades 12, these air jets 18 being indicated by corresponding arrows in FIG. 1. The air jets 18 discharge from the suction side 21 of the blade 12 and, in interaction with the flow 17, cause vortices there which extend in the direction of flow. The driving force for the air jets 18 is the pressure difference between the pressure side 20 and the suction side 21. The static pressure ratio between the pressure side 20 and the suction side 21 can be estimated as 1.4. This value increases if, in addition to the static pressure, a dynamic pressure on the pressure side 20 is also utilized, as will be explained further below.

The cross section through a blade 12 according to FIG. 1 at the level of an air jet is shown in an enlarged detail in FIG. 2. From the pressure side 20 of the blade 12, a first passage or a bore 22, under a slight inclination at an angle W1 of 20° relative to the pressure-side blade surface, passes into the interior of the blade 12. In the pressure-side surface, the outer end of the first passage 22 forms an opening 26, through which the air enters the blade 12 from the pressure side 20. The first passage 22 preferably has a diameter of about 1 mm. The first passage 22 ends in the interior of the blade 12. Starting from the inner end of the first passage 22, a second, steeper passage 23 runs at an angle W2 of about 60° to the pressure-side blade surface up to the suction-side blade surface ends there with an opening 25, through which the air discharges as air jet 18. The diameter of the second passage or of the second bore 23 is about 0.4 mm. If the passages 22, 23 are viewed in the direction of the arrows A depicted in FIG. 2, the view reproduced in FIG. 3 is obtained. Whereas the projection of the first passage 22 runs perpendicularly downwards, the projection of the second passage is inclined to the side by about 45°, so that the air jets not only discharge obliquely forward in the direction of flow but are also tilted toward one side. The result of this inclination of the first passage 22 is that, in addition to the static pressure, the dynamic pressure on the pressure side 20 of the blade is also absorbed and utilized for producing the air jets.

As shown by the side view, reproduced in FIG. 4, of the blade 12 with the blade tip 24 at the top, a row of first openings 25 with corresponding air feed from the pressure side are arranged in the radial direction on the suction side. The first openings 25 are evenly spaced apart at a preferred distance a of about 4 mm.

An essential feature of the solution shown is an arrangement of a plurality of passages through the blade from the pressure side to the suction side in a row which extends in the radial direction. The special advantages of the solution are:

-   -   No additional parts are introduced here, which could separate         from the blade and cause damage.     -   Air under pressure is supplied at the blade automatically by the         pressure difference between pressure side and suction side.     -   No external control and monitoring mechanism is required.     -   The solution can easily be implemented subsequently.     -   The working range of the compressor and the pressure ratio are         markedly increased in a simple manner.         List of Designations

-   10 Axial-Flow Compressor

-   11 Blade ring

-   12 Blade

-   13 Leading edge (blade)

-   14 trailing edge (blade)

-   15 Axial direction

-   16 Direction of rotation

-   17 Flow

-   18 Air jet

-   19 Shock wave front

-   20 Pressure side (blade)

-   21 Suction side (blade)

-   22, 23 Passage

-   24 Blade tip

-   25, 26 Opening

-   a Distance

-   W1, W2, W3 Angle 

1. A method of improving the flow conditions on the suction side of the blade of an axial-flow compressor provided for compressing air, in which method vortices running in the direction of flow are generated on the suction side, wherein the vortices are generated by local air jets formed on the suction side.
 2. The method as claimed in claim 1, wherein, to form the local air jets, air having a higher pressure relative to the environment is discharged from first openings in the suction-side surface of the blade.
 3. The method as claimed in claim 2, wherein the air having the higher pressure on the pressure side of the blade is extracted from the flow and is directed through the blade to the first openings in the suction-side surface of the blade.
 4. The method as claimed in claim 3, wherein a second opening on the pressure side of the blade is assigned to each first opening on the suction side of the blade, and wherein the air having the higher pressure is in each case extracted at the second opening and directed via passages in the blade to the associated first opening.
 5. The method as claimed in claim 3, wherein, to increase the pressure of the air extracted at the pressure side, the dynamic pressure associated with the flow is also utilized in addition to the static pressure prevailing on the pressure side.
 6. The method as claimed in claim 5, wherein a first passage in the blade is used in order to utilize the dynamic pressure during the extraction of the air, this first passage being inclined in the direction of flow.
 7. An axial-flow compressor for carrying out the method as claimed in claim 1, which axial-flow compressor comprises at least one blade ring of blades extending in the radial direction, wherein each of the blades on the suction side has a row of first openings extending in the radial direction, these first openings each being connected to a source for air having a higher pressure.
 8. The axial-flow compressor as claimed in claim 7, wherein the first openings are each arranged in the vicinity of the leading edge of the blade.
 9. The axial-flow compressor as claimed in claim 7, wherein, as a source for air having a higher pressure, second openings on the pressure side of the blade are assigned to the first openings on the suction side of the blade.
 10. The axial-flow compressor as claimed in claim 9, wherein a second opening is assigned to each first opening and is connected to the first opening via passages running transversely through the blade.
 11. The axial-flow compressor as claimed in claim 10, wherein in each case a first passage is arranged in the interior of the blade behind the second opening, this first passage running in the direction of flow and enclosing an acute angle, in particular of about 20°, with the pressure-side surface of the blade.
 12. The axial-flow compressor as claimed in claim 11, wherein the first passage has a diameter of about 1 mm.
 13. The axial-flow compressor as claimed in claim 11, wherein in each case a second passage is arranged in the interior of the blade behind the first opening, this second passage running in the direction of flow and enclosing an acute angle, in particular of about 60°, with the pressure-side surface of the blade.
 14. The axial-flow compressor as claimed in claim 13, wherein the second passage is tilted by an angle, preferably of about 45°, from the plane spanned by the first passage and the direction of flow.
 15. The axial-flow compressor as claimed in claim 13, wherein the second passage has a diameter of about 0.4 mm.
 16. The axial-flow compressor as claimed in claim 13, wherein the first and second passages open into one another in the interior of the blade.
 17. The axial-flow compressor as claimed in claim 7, wherein the first openings of each blade are uniformly spaced apart and are at a distance from one another of about 4 mm. 